Rotor position information is in general required for the stable operation of permanent magnet AC motors having sinusoidal current excitation. Continuous rotor position has been obtained in the past from encoders mounted on the motor shaft or indirectly through estimation algorithms based on voltage and current feedback. The latter is preferred because it results in lower system and operating cost.
However, most passive rotor estimation schemes (based on measured voltage and current) are complex and require precise knowledge of the motor parameters such as resistance and inductance. These parameters, particularly the stator resistance, change widely with temperature. This leads to inaccuracy in rotor angle estimation and results in control stability problems, reduced torque per ampere capability and degradation of motor operating efficiency.
Ser. No. 10/294,201 describes a method wherein the rotor angle is estimated via a phase-lock loop (PLL), which locks on the motor's magnetic flux, particularly during the normal running mode.
However, the inventor has observed that additional problems arise during start-up. At zero speed or low speed (<10%) conditions, it is difficult to accurately measure or estimate motor voltages due to the low amplitude of motor back EMF (BEMF). In most sensor-less (having no shaft encoder) control drives, the tracking of rotor angle based on BEMF normally fails at low speeds (<5%). Therefore, sensor-less control of a permanent magnet motor drive requires some means of starting the motor. In most cases, the motor is started in an open-loop (without use of any feedback) fashion. As soon as the motor speed picks up (typically>10%), the drive switches to closed-loop (using current and/or voltage feedback) control mode. However, during the switchover from open loop to closed-loop mode, torque and current pulsation may occur due to mode transitioning.
It would therefore be desirable to provide a rotor angle estimation scheme which provides maximum torque per ampere performance without requiring accurate knowledge of the stator resistance or other motor parameters.
It would further be desirable to provide a scheme for estimating rotor angle during start-up so as to provide for robust start-up and reduce the occurrence of torque pulsation during start-up of the motor.